Antistatic powder coating compositions and their use

ABSTRACT

This invention relates to a powder coating composition comprising a mixture of conventional, possibly coloured, non-conductive thermosetting powder coating compositions with highly conductive thermosetting powder coating compositions so as to produce a coating having an electrical surface resistance of less than 10 10  Ω (ohm), preferably of less than 10 8  Ω (ohm), and to the substrates coated therewith. This invention also relates to using a mixture of non-conductive and conductive powder coating compositions to produce a coated surface having an electrical surface resistance sufficiently low to possess antistatic properties.

BACKGROUND OF THE INVENTION

This invention relates to a powder coating composition, its use for thepreparation of coated surfaces with antistatic properties and the coatedsubstrates.

Thermosetting powder coatings are applied as protective or decorativefinishes in a variety of applications. Easy application, low emissionsand low waste materials generation are key advantages of powdercoatings. Powder coatings are normally applied by electrostatic sprayprocesses. The powder is charged by friction or by a corona dischargeand then applied to the substrate where it adheres by electrostaticforces. The substrate is heated to temperatures above the softeningpoint of the powder coating. The powder coating then melts and forms acontinuous film on the substrate. On further heating, the crosslinkingreaction of the coating composition is initiated. After cooling, adurable, flexible coating is obtained.

In certain applications it is desirable that coated surfaces have a lowelectrical surface resistance to provide antistatic or even electricalconductive properties. Examples are furniture used in assembly areas forelectronical equipment, furniture or equipment used in explosion proofareas, containers or housing for electronic equipment etc. Standards forthe surface conductivity of materials that are used in areas wheredevices are handled which are sensitive to electrostatic discharge canfor instance be found in the European Standard EN 100 015-1 or theSwedish Standard SP-Method 2472.

Conventionally, the antistatic or electrical conductive properties areobtained by application of liquid paint that contains high amounts ofconductive additives like carbon black, specially coated pigments ormetallic powders.

Many attempts have been made to increase the electrical conductivity ofpowder coatings to allow the use of powder coatings in the areasmentioned above. For instance, patent application CN 1099779 describesthe addition of conductive micro particles like graphite, acetyleneblack or zinc oxide in relatively high concentrations to powder coatingformulations. This procedure has the disadvantage that the coatingscontaining graphite or acetylene black generally do not allow toformulate light coloured coatings. On the other hand, high amounts oflight coloured fillers like zinc oxide give coatings with poorreproducibility of the electrical conductivity (see “N. G. Schibrya et.al.” Antistatic decorative coatings based on coating powders “Electron”,Russia Lakokras. Mater. Ikh Primen. (1996) (12), page 19-20 andreferences cited therein). In this reference, conductive coatings withvery high loads of metallic powders are disclosed. Such powder coatingcompositions have a very high specific gravity and are thereforedifficult to apply to a substrate. In addition, the use of fine metalpowders imposes safety risks during the powder manufacturing andapplication.

The German patent application DE-A-198 09 838 claims the addition ofconductive polymeric materials to increase the electrical conductivityof powder coated surfaces. Again, these polymers have a black or darkcolour and result in dark coloured powder coatings.

In the U.S. patent U.S. Pat. No. 4,027,366, the application of mixturesof powders that have differences in the dielectric constant is describedwhereby one powder material is a conductive metal or nonmetal. Theobject of U.S. Pat. No. 4,027,366 is to produce multi layer coatings inone step. The preparation of coatings with antistatic properties or alow electrical surface resistance of the coated surface is notmentioned.

Accordingly, it is the object of the invention to provide powdercoatings that have a low and reproducible electrical surface resistanceof the coated substrate surface, can be prepared in various colours andare easy to apply even with variation in the coating film thickness.

SUMMARY OF THE INVENTION

This object is achieved by a mixture of conventional, possibly coloured,non-conductive thermosetting powder coating materials with highlyconductive thermosetting powder coating materials. The ratio of theconductive to the non-conductive powder coating materials in theinventive mixture can be between 2.5 to 95 and 95 to 2.5. The ratio canbe adjusted to meet the requirements for the electrical surfaceresistance in a specific application. Generally, a higher percentage ofthe conductive component in the blend yields a lower electrical surfaceresistance of the final coating. The mixture according to the inventionresults in coloured coatings that have an electrical surface resistanceof less than 10¹⁰ Ω (ohm), preferably of less than 10⁸ Ω (ohm). Thissurface resistance is sufficiently low for many applications thatrequire antistatic properties of a surface.

DETAILED DESCRIPTION OF THE INVENTION

The non-conductive thermosetting powder coating material in theinventive mixture can be any thermosetting powder coating composition.The powder can be coloured or transparent, e.g. clear coat.

The powder compositions which may be used for example, are those basedon polyester resins, epoxy resins, polyester/epoxy hybrid resin systems,(meth)acrylic resins, polyurethane resins. Suitable crosslinking resinsfor the binder/hardener system are, for example, di- and/orpolyfunctional epoxides, carboxylic acids, dicyandiamide, phenolicresins and/or amino resins, in the usual quantity. The compositions maycontain constituents conventional in powder coating technology, such aspigments and/or fillers and further additives.

Suitable powder coating formulations are for instance described in D. A.Bates “The Science of Powder Coatings” Volume 1, Sita Technology,London, 1990. Surfaces that are coated with such powder coatingmaterials generally have an electrical surface resistance of greaterthan 10¹⁰ Ω (ohm).

The conductive thermosetting powder coating composition of the inventivemixture contains contains high concentrations of inorganic or organicconductive fillers and/or pigments. Such fillers and/or pigments may forinstance be carbon black, conductive polymeric materials or lightcoloured inorganic pigments. If carbon black or conductive polymericmaterials are used, the conductive powder materials are generally blackor dark coloured. Examples for conductive polymeric materials arepolyaniline, polypyrole or polythiophene or their derivatives. For lightcoloured conductive powder coatings may be used metal oxides, non-metaloxides, conductively coated barium sulphate or potassium titanate, dopedtin dioxide, doped zinc oxide (doped for example with aluminium,gallium, antimony, bismuth), or special inorganic pigments can be used.Examples of such special inorganic pigments are metal oxide coated micaplatelets like zinc oxide coated mica, antimony doped tin oxide coatedmica and which are given in R. Vogt et. al “Bright conductive pigmentswith layer substrate structure”, European Coatings Journal, page 706,1997. For economic reasons, it is preferred to use carbon black asconductive filler. The conductive powder coating material useful for themixtures according to the invention contains between 1 and 20 weight %,preferably between 2 and 10 weight % of the conductive fillers and/orpigments. It is also possible to use mixtures of different conductivefillers and/or pigments to formulate the conductive powder coatingmaterial. In general, coatings prepared from the conductive powdercoating without the addition of non-conductive powders should have anelectrical surface resistance of less than 10⁶ Ω(ohm) or at least by afactor of 10 lower than the desired surface resistance of the surfaceswhere the mixture according to the invention was applied to. This meansthat the conductive component must for example have an electricalsurface resistance of less than 10⁷ Ω (ohm) if the mixture withnon-conductive powders should have a surface resistance of less than 10⁸Ω.

The binder/hardener system of the conductive powder material can be thesame as for the none conductive powder material in the mixture accordingto the invention or it can be different. For smooth finishes, it ispreferred to use the same binder/hardener system in the conductive andthe non conductive powder material of the mixture.

It is possible to match the colour of the non-conductive material of thepowder mixture and the conductive powder if light coloured conductivepigments or fillers are used to generate a uniform coating colour.However, it is also possible and preferred to use a black conductivepowder in combination with non conductive powders that have a differentcolour e.g. white, grey, red or yellow. Such mixtures will generate acoating with a sprinkle effect because the individual colours of thepowders will be more or less visible to the human eye. Such coatingsyield attractive finishes that are suitable for many applications.

The conductive and the non-conductive thermosetting powder coatingmaterials that are required for the mixture according to the inventioncan be prepared by known powder coating manufacturing technologies, forinstance by known extrusion/milling techniques, by spay processes e.g.from supercritical solutions, or by melt atomisation, or bysuspension/dispersion processes, e.g. non-aqueous dispersion process.

The powder materials useful for the mixture according to the inventionhave for example a mean particle size from 10 to 100 μm, preferably from15 to 50 μm. The conductive and the non conductive powder materials canhave the same mean particle size and the same particle sizedistribution. It is also possible to mix powder materials that havedifferent mean particle sizes or different particle size distributions.A similar particle size distribution for all components of the inventivemixture is preferred. It is also preferred to select powder materialsfor the mixture that have a similar specific gravity.

The mixture according to the invention can be prepared by standardmixing devices that deliver homogeneous mixtures of powders like tumblemixers, high shear rotating blade mixers or continuous mixers. It isalso possible to use special mixers potentially at slightly elevatedtemperatures that are normally used to bond pigments to powder coatings.If such mixers are used, the different powder materials are at leastpartially bonded to each other which may be advantageous for certainapplications.

The inventive mixture can be applied to various substrates like metals,plastics, wood or wood composites by known powder applicationtechnologies, for instance by electrostatic spray processes using coronaor tribo charge. It is also possible to apply the powder mixture in theform of an aqueous dispersion or a powder slurry.

The substrate is then heated by suitable means to temperatures thatallow the powder coating to flow out and cure. The temperature and timerequired for melting and curing will depend on the binder/hardenersystem used in the powder coating formulation. Typical conditions arefor example temperatures of 160° C. for 20 minutes if convection ovensare used to heat up the substrate. Significantly shorter time periodscan be accomplished if infra red or near infra red (NIR) radiation isused to melt and cure the powder coatings. It is also possible toformulated powder coatings that can be cured by UV-radiation. In thiscase it is preferred that both, the conductive and the non conductivematerial of the powder mixture can be cured by UV-radiation.

Typical film thicknesses of the coating after curing are for examplebetween 20 and 150 μm. It is a special advantage of the presentinvention that the surface resistance of the coating is not sensitive tothe film thickness which is critical in many applications wherevariations in film thickness cannot be avoided. In general, a coatingthickness between 50 and 100 μm is preferred.

The powder coatings according to the invention provide surfaces invarious colours with an attractive smooth finish and a low andreproducible surface resistance suitable for the use as antistaticcoating. The degree of gloss can be adjusted by known powder coatingtechnology.

The following examples further illustrate the invention:

The electrical surface resistance of the coatings was measured with the“Test Kit for Static Control Surfaces” supplied by 3M which meets therequirements of the standard EOS/ESD-S4.1-1990.

Preparation of a Conductive Powder Coating

EXAMPLE 1

A mixture consisting of 16 wt.-% epoxy resin, 42 wt.-% polyester resin,37 wt.-% barium sulphate, 3.5 wt.-% carbon black and 1.5 wt.-% flow anddegassing agents is intimately blended, and extruded at a temperaturebetween 110 and 140° C. The extrudate is ground into a fine black powderwith a mean particle size of 38 μm.

The powder material is applied by electrostatic spray application to asteel panel and is then cured for 10 minutes at 200° C. The coatingobtained has an electrical surface resistance of 10⁴ to 10⁵ Ω (ohm)measured at 100V and a coating thickness of 90 μm.

Mixtures According to the Invention

EXAMPLE 2

The conductive powder material from example 1 is intimately mixed in atumble mixer with a commercial blue polyester/epoxy hybrid powder in aratio of 20/80 (weight by weight). The resulting mixture is applied byelectrostatic spray application to a steel panel and then cured for 10minutes at 200° C. A blue smooth finish with black sprinkles is obtainedthat has a surface resistance of 10⁵ to 10⁶ Ω (ohm) when measured at100V and a coating thickness of 90 μm.

EXAMPLE 3

The conductive powder material from example 1 is intimately mixed in atumble mixer with a commercial red polyester/epoxy hybrid powder in aratio of 30/70 (weight by weight). The resulting mixture is applied byelectrostatic spray application to a steel panel and the coating is thencured for 10 minutes at 200° C. A red smooth finish with black sprinklesis obtained that has a surface resistance of 10⁵ to 10⁶ Ω (ohm) whenmeasured at 100V and coating thicknesses of 60 μm and 100 μm.

What is claimed is:
 1. A powder coating composition comprising a mixtureof at least one conductive thermosetting powder coating compositioncomprising at least one conductive filler selected from carbon black,and at least one non-conductive thermosetting powder coating compositioncomprising a non-black color; wherein the weight ratio of the at leastone conductive thermosetting powder coating composition to the at leastone non-conductive thermosetting powder coating composition is between2.5 to 95 and 95 to 2.5; wherein said powder coating compositionproduces a coating having an electrical resistance of less than 10¹⁰ohm.
 2. The powder coating composition of claim 1 wherein a cured layerof the at least one conductive thermosetting powder coating compositionhas an electrical surface resistance of less than 10⁶ ohms.
 3. Thepowder coating composition of claim 1 wherein a cured layer of the atleast one non-conductive thermosetting powder coating composition has anelectrical surface resistance greater than 10¹⁰ ohms.
 4. The powdercoating composition of claim 1 wherein the at least one conductivethermosetting powder coating composition comprises 1-20 wt. % of thecarbon black conductive filler.
 5. The powder coating composition ofclaim 1 wherein the conductive thermosetting powder coating compositionfurther comprises at least one conductive polymeric material selectedfrom polyaniline and derivatives thereof, polypyrole and derivativethereof, polythiophene and derivatives thereof, and mixtures thereof. 6.The powder coating composition of claim 5 wherein the conductivethermosetting powder coating composition contains from 1-20 wt. % of thecarbon black conductive filler and/or the at least one conductivepolymeric material.
 7. The powder coating composition of claim 1 curableby high energy radiation selected from the group consisting of infraredradiation, near infrared radiation and UV radiation.
 8. A substratecoated with a cured layer of the powder coating composition of claim 1.9. The coated substrate of claim 8 wherein the surface properties of thelayer are antistatic properties.
 10. A light colored powder coatingcomposition comprising a mixture of at least one conductivethermosetting powder coating composition comprising at least oneconductive light colored inorganic pigment selected from metal oxides,non-metal oxides, conductively coated barium sulphate, conductivelycoated potassium titanate, doped tin dioxide, doped zinc oxide, metaloxide coated mica platelets, and mixtures thereof; and at least onenon-conductive thermosetting powder coating composition; wherein theweight ratio of the at least one conductive thermosetting powder coatingcomposition to the at least one non-conductive thermosetting powdercoating composition is between 2.5 to 95 and 95 to 2.5; wherein saidpowder coating composition produces a light-colored coating having anelectrical resistance of less than 10¹⁰ ohm.
 11. The powder coatingcomposition of claim 10 wherein a cured layer of the at least oneconductive thermosetting powder coating composition has an electricalsurface resistance of less than 10⁶ ohms.
 12. The powder coatingcomposition of claim 10 wherein a cured layer of the at least onenon-conductive thermosetting powder coating composition has anelectrical surface resistance greater than 10¹⁰ ohms.
 13. The powdercoating composition of claim 10, wherein the conductive thermosettingpowder coating composition comprises 1-20 wt. % of the least oneconductive light colored inorganic pigment.
 14. The powder coatingcomposition of claim 10, wherein the at least one conductivethermosetting powder coating composition further comprises at least oneconductive polymeric material selected from polyaniline and derivativesthereof, polypyrole and derivatives thereof, polythiophene andderivatives thereof, and mixtures thereof.
 15. The powder coatingcomposition of claim 14, wherein the conductive thermosetting powdercoating composition contains 1-20 wt. % of the at least one conductivelight colored inorganic pigment and/or the at least one conductivepolymeric material.
 16. A process for preparing a powder coatingcomposition comprising mixing at least one conductive thermosettingpowder coating composition comprising at least one conductive fillerselected from carbon black with at least one non-conductivethermosetting powder coating composition comprising a non-black color;wherein the weight ratio of the at least one conductive thermosettingpowder coating composition to the at least one non-conductivethermosetting powder coating composition is between 2.5 to 95 and 95 to2.5.
 17. The process according to claim 16, wherein said powder coatingcomposition produces a coating having an electrical resistance of lessthan 10¹⁰ ohm.